Oxide-based quantum cutter method and phosphor system

ABSTRACT

A method of producing two visible light photons from an oxide-based phosphor doped with praseodymium (Pr) and including atoms of at least one activator in response to excitation with a single ultraviolet light photon. The method includes exciting the Pr of the oxide-based phosphor with a photon of ultraviolet light to excite an electron to an excited state, the excited electron falling to a lower energy state in a non-radiative transition and transferring energy to excite a first activator atom in the oxide-based phosphor, the first activator atom emitting a first photon of visible light, and the excited electron falling further to a lower energy state in a non-radiative transition, transferring energy to excite a second activator atom in the oxide-based phosphor, the second activator atom emitting a second photon of visible light. An oxide-based phosphor doped with praseodymium includes atoms of at least one activator and emitting two visible light photons in response to excitation by a single ultraviolet light photon.

FIELD OF THE INVENTION

[0001] The present invention is directed to an oxide-based phosphor anda process including a oxide-based phosphor doped with praseodymium (Pr)and including at least one activator so that pumping with one photon ofultraviolet (UV) light produces two photons of visible light. Theinvention is particularly applicable to, but it not limited to, plasmadisplay panels (PDP). These panels have gained increasing acceptance indisplay technology.

BACKGROUND

[0002] In the typical PDP, electrical discharges are produced inrespective areas, i.e., pixels, addressed by electrical signals. Thedischarges occur within gases contained within the PDP, typically zenonand helium. The plasma discharge within these gases typically producesultraviolet light, particularly vacuum ultraviolet (VUV) light withcharacteristic wavelengths at 147 nm and 172 nm which correspond tophoton energies of 8.3 and 7.2 eV, respectively. This invisible lightmust be converted to visible light in order to produce an image on aPDP.

[0003] Typically, phosphors are employed for the conversion of the VUVlight to visible light. A similar light energy conversion process isused in fluorescent lamps but is not desirable in PDPs becausefluorescent lamps employ mercury which has delayed emissioncharacteristics and adverse environmental considerations. Further, in aPDP, it is important that the phosphors produce colors to reproduce acolor image. Many lamp phosphors have been successfully adapted to PDPsby enhancing the VUV light absorption properties and response times.However, the large energy difference between the pumping light and theemitted photons results in very low energy efficiency.

[0004] One approach to improving the energy efficiency is the use of aquantum cutter phosphor in which a high energy photon produces two lowerenergy photons. This process has been demonstrated in phosphors withfluoride-based lattices, such as YF₃ and NaYF₄. Quantum efficiencies forthese quantum cutter phosphors exceeding 100% for visible light, i.e.,light having wavelengths between 400 and 705 nm, have been reported. Thequantum cutting phenomenon has been observed in an oxide phosphorlattice of SrAl₁₂O₁₉ doped with Pr. Nevertheless, the visible lightquantum efficiency for this oxide-based lattice is relatively lowbecause an energy transition producing a UV emission competes with thequantum cutting energy transitions that produce visible light. Otherschemes employing quantum cutting in phosphors of LiGdF₄ and GdF₄ dopedwith europium (Eu) and producing two photons of the same color, i.e.,red, with a conversion efficiency as high as 195% have been reported.The production of two photons with the same energy is a significantadvantage over previous achievements with Pr-doped phosphors employingquantum cutting in which the two produced photons cover a wide spectralrange, from infrared (IR) to UV.

[0005] In previous work with Pr-doped fluoride-based phosphor lattices,for example, LaF₃ and NaYF₄, the light emissions include a principalline having a wavelength at 407 nm and a second emission more clearlywithin the visible range and having a wavelength between 470 and 620 nm.An example is illustrated in FIG. 1. The energy levels and transitionsinvolved in this process are schematically illustrated in FIG. 2. Anelectron of a Pr³⁺ ion is excited to a 4f5d band by the absorption of anincident pumping UV photon. The exciter electron non-radiatively decaysto the ¹S₀ state. Then, the emitted light having a wavelength of 407 nmis produced from a ¹S₀-¹I₆ transition, followed by another non-radiativeenergy loss of the electron into the ³P₀ level followed by a secondaryradiative transition to various ³F and ³H levels. The latter transitionproduces visible luminescence at wavelengths between 420 nm and 650 nm.However, unless appropriate steps are taken to isolate some of theenergy levels from the broad 4f5d band, the emission spectrum will bedominated by an intense UV emission. To avoid that result, a latticewith a weak crystalline field, such as a fluoride-based lattice, hasbeen necessary to produce a useful quantum cutter phosphor. However, thefluoride-based lattices are unstable, preventing practical applicationsof those materials to PDPs and other applications, for example,lighting.

[0006] Oxide-based lattice phosphors are stable but have relativelystrong crystalline fields. However, in oxide-based lattices with highcoordination numbers, the crystalline field can be relatively weak. Inaddition, crystalline fields are expected to be reduced in strength inoxide-based lattices when doped with Pr, producing Pr³⁺ ions within thelattice. It is on that basis that an SrAl₁₂O₁₉ phosphor lattice dopedwith Pr exhibiting the quantum cutting phenomenon has been demonstrated.However, the quantum efficiency of this oxide-based lattice phosphorfails to achieve the efficiencies observed for fluoride-based latticesand, therefore, have not been satisfactory.

SUMMARY OF THE INVENTION

[0007] Accordingly, it is an objective of the invention to provide anoxide-based lattice phosphor that is stable, that has a relatively weakcrystalline field due to the presence of Al, that has a highcoordination number, that can be doped with Pr, and that exhibits thequantum cutting phenomenon in which two visible photons are produced inresponse to a single high energy UV pumping photon.

[0008] It is a further objective of the invention to produce such aphosphor having a quantum efficiency exceeding that previously observedwith such oxide-based lattice phosphors and, preferably, which exceedsthe quantum efficiency of fluoride-based lattice phosphors.

[0009] A method of producing two visible light photons from anoxide-based phosphor doped with praseodymium and including atoms of atleast one activator in response to excitation with a single ultravioletlight photon according to the invention includes exciting the Pr of theoxide-based phosphor with a photon of ultraviolet light to excite anelectron to an excited state, the excited electron falling to a lowerenergy state in a non-radiative transition and transferring energy toexcite a first activator atom in the oxide-based phosphor, the firstactivator atom emitting a first photon of visible light, and the excitedelectron falling further to a lower energy state in a non-radiativetransition, transferring energy to excite a second activator atom in theoxide-based phosphor, the second activator atom emitting a second photonof visible light.

[0010] The first and second photons have the same wavelength.

[0011] An oxide-based phosphor doped with praseodymium according to theinvention includes atoms of at least one activator and emitting twovisible light photons in response to excitation with a singleultraviolet light photon.

[0012] The activator atoms according to the invention are chosen frommanganese, terbium, and europium.

[0013] A preferred oxide-based phosphor lattice is SrAl₁₂O₁₉.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a graph showing the photoluminescence spectrum of aprior art YF₃ lattice phosphor doped with Pr.

[0015]FIG. 2 is an energy level diagram of the energy levels of Pr³⁺ions illustrating quantum cutting energy transitions.

[0016]FIG. 3 is a three-part energy level diagram indicating energylevel transitions providing quantum cutting for a Pr-doped oxide latticeincluding Mn activator atoms according to an embodiment of theinvention.

[0017]FIG. 4 shows photoluminescence spectra of a oxide-based latticephosphor doped with Pr exhibiting the quantum cutting phenomenonaccording to the invention.

[0018]FIG. 5 is a graph showing the photoluminescence intensity ofemitted light with a wavelength of 401 nm as a function of Pr dopantconcentration in an oxide-based lattice phosphor according to theinvention.

[0019] FIGS. 6(a) and 6(b) are graphs of photoluminescence intensitywith respect to the concentration of Pr in an oxide-based latticephosphor according to the invention including respective activatoratoms.

[0020]FIG. 7 is a photoluminescence spectrum of the Pr-doped oxide-basedlattice phosphor according to the invention showing emission at awavelength of 401 nm.

[0021]FIG. 8 shows photoluminescence excitation spectra for anoxide-based lattice phosphor with only Pr doping, with doping of both Prand Mn, and with only Mn doping.

DETAILED DESCRIPTION

[0022] In the invention, an oxide-based lattice phosphor was developedusing Pr doping in order to produce Pr³⁺ ions in a lattice having aweakened internal crystalline field. Further, the lattice phosphor wasdesigned so that resonant transfers of energy between atoms was avoided.Such resonant energy transfers increase energy transfer probabilitiesbut seriously limit the possibility of developing phosphors employingquantum cutting that emit different colors of light. To achieve thedifferent colors, similar resonant energy transfer mechanisms have to befound, severely limiting the design of the phosphor.

[0023] To achieve the desired energy transfers in the oxide-basedlattice phosphor with the weakened internal crystalline field,activating atoms were added to the lattice for non-resonant energytransfer between excited electrons of the Pr³⁺ ions and the activators.In this design, it is essential that two step energy transitionsproducing the quantum cutting phenomenon of the Pr³⁺ ions be maintainedwithout interference while achieving efficient coupling between the Pr³⁺ions and the activator atoms.

[0024] We concluded that good choices for activators are manganese (Mn),terbium (Tb), and europium (Eu). An important feature of the resultinglattices is that the Pr doping with Mn activator atoms and the Pr dopingwith Tb activator atoms employ non-resonant energy transfers, bycontrast with previously known systems, such as the Gd-Eu system, thatexploit resonant energy transfers.

[0025] An example of the energy transfers that can occur in anoxide-based lattice phosphor doped with Pr and containing Mn activatoratoms is schematically illustrated in FIG. 3. As illustrated in thecentral part of that figure, UV light having a wavelength of 195 nmexcites an electron of a Pr³⁺ ion to the 5d energy level. That electronquickly relaxes into the ¹S₀ energy level, which is the initial statefor the two transitions constituting the quantum cutting phenomenon thatproduces two visible light photons. In the first step of thesetransitions, the excited electron makes an ¹S₀-¹I₆ transition, radiatinglight having a wavelength of 401 nm. This transition is followed byother transitions, ³P₀-³F, ³H producing photons of visible light. InFIG. 3, the first transition is labeled A and transfers energy to anMn²⁺ ion when Mn is the activator atom in the lattice. The Mn latticetransition then produces the first photon of the phenomenon, in thiscase light having the color green. Thereafter, the Pr³⁺ ion furtherrelaxes, producing an energy transfer to a second of the activatoratoms, i.e., Mn²⁺ ions, labeled B in FIG. 3, to produce a second photonof green light. In other words, in response to the incidence of thepumping UV photon, an electron of a Pr³⁺ dopant atom is excited and,after some relaxation, falls in energy, emitting light and exciting afirst Mn activator atom that, in turn, emits a visible photon.Thereafter, the electron transfers energy to a second activator atom andreturns to a lower energy state without radiation. The second activatoratom then radiates a second photon having the same wavelength as thephoton radiated by the first activator atom.

[0026] In order to confirm the design of the oxide-based latticephosphor, a host material of SrAl₁₂O₁₉ was synthesized in a solid statereaction. Strontium carbonate was stoichiometrically mixed with aluminumoxide and doped with Pr₂O₅ to supply Pr. Magnesium carbonate was addedso that the magnesium could function as a charge compensator. Bothaluminum fluoride and ammonium fluoride were used as fluxes in firingthe mixture at a temperature of 1,350° C. for about two hours in amixture of argon and hydrogen. It was determined that a second firing ina reducing atmosphere after the initial firing improved quantumefficiency. Approximately a 50% improvement in brightness of the emittedlight was achieved when the second firing process was employed. Afurther, 10%, improvement was achieved when the flux was ammoniumfluoride rather than aluminum fluoride, probably due to the eliminationof residual aluminum compounds in the phosphor. Still higher brightnesswas achieved when the strength of the reducing atmosphere was increasedby including graphite powder within the precursor mixture in a 5%hydrogen atmosphere. These conditions resulted in better activation ofthe Pr ions, yielding the improved brightness. The samples producing thegreatest brightness included 4% Pr.

[0027]FIG. 4 is a photoluminescence spectrum of a Pr-doped SrAl₁₂O₁₉phosphor including Mn as an activator. The figure shows the dominantlight emission at a wavelength of 401 nm due to the initial ¹S₀-¹I₆energy transition of the Pr³⁺ ions. The second largest band of lightemission occurred at a wavelength of 486 nm, corresponding to the³P₀-³H₄ energy transition. Since the ³P₀ of the second transitionproducing the light of wavelength 486 nm was populated through the firstenergy transition (¹S₀-¹I₆), followed by a non-radiative relaxation, thesimultaneous observation of light wavelengths corresponding to ¹S₀-¹I₆and ³P₀-³H₄ directly confirms the existence of the quantum cuttingphenomenon. The success of this quantum cutter lattice in producing thedesired two visible light photons critically depends upon maximizing theprobability of the ¹S₀-¹I₆ energy transitions with regard to competingenergy transitions, particularly a transition that produces relativelystrong light emission at a wavelength of 273 nm. This result is achievedin the invention because the ratio of visible light to UV light wasobserved to be 11.5, significantly larger than the same ratio for afluoride-based lattice, for example, YF₃ doped with Pr. Thus, a quantumefficiency exceeding 100% is expected for oxide-based lattice phosphorsaccording to the invention.

[0028] Our experiments demonstrated that the Pr concentration producinga maximum radiation at the 401 nm wavelength was 4%, with a decline inthe intensity of that wavelength emission as the concentration of Princreased. Further, it appeared that the addition of magnesium as aco-dopant for charge compensation within the oxide-based latticephosphor was very important in realizing high quantum efficiency.Although attempts to replace some of the Sr atoms with Ba, Ca, Zn, andLa were made, the luminescence intensity decreased when thesesubstitutions were made, probably because the oxide-based latticephosphors tested were not monocrystalline. Monocrystalline latticephosphors should produce much higher quantum efficiencies.

[0029] Oxide-based lattice phosphors respectively including Mn and Tb asactivator atoms were prepared. FIG. 6(a) is a graph of measuredphotoluminescence spectra for lattices including Mn, illustrating thechange in luminescence spectra as a function of Pr concentration. FIG.6(b) shows photoluminescence spectra for a lattice including Tb as theactivator atom. Both figures show that the Pr luminescence decreasedrapidly with increased Pr concentration, probably due to the formationof secondary phases at high Pr concentrations. These spectra suggestthat monocrystalline phosphors should be used at high Pr concentrations.The figures also show that the Mn and Tb emission intensities remainessentially unchanged even when their concentration doubles. Theseresults suggest that only a limited amount of these activators are beingincorporated into the lattice to participate in the quantum cuttingphenomenon. Further enhancements in processing should increase theconcentration of active dopants in activators, with a correspondingincrease in external quantum efficiency, exceeding 100%.

[0030] Proof that quantum cutting is occurring in the Pr-dopedoxide-based lattice phosphors according to the apparatus is obtained bysimultaneously observing the ¹S₀-¹I₆ transition, producing light at awavelength of 401 nm, and lower energy transitions which, for example,produce green light at a wavelength of 480 nm, in the case of using Mnas the activator, and red light at a wavelength of 610 nm, in the caseof using Tb as the activator.

[0031]FIG. 7 shows the photoluminescence spectrum of Pr-doped SrAl₁₂O₁₉in which light emissions at 401 nm and green light at 480 nm wereobserved. This spectrum unambiguously proves that quantum cuttingaccording to the phosphor design is occurring. In our experiments, thetested material exhibited a total visible quantum efficiency greaterthan that of a commercially available phosphor, illustrating thecommercial potential of this system.

[0032] Photoluminescence excitation (PLE) spectroscopy may be used tomonitor the excitation process specific to an ion. In other words, PLEmay be employed to identify an energy transfer process involved in theexcitation of a certain type of ion, for example, Mn, when used as theactivator. Three PLE spectra are shown in FIG. 8. In the first of thosespectra, the energy transition of the Pr³⁺ ion producing the 401 nmwavelength emission exhibited two excitation bands, one centered at awavelength of 195 nm and a second centered at a wavelength of 160 nm.The first excitation band is attributable to direct absorption of lightby the 5d energy levels within the Pr³⁺ ions and the second band is dueto host absorption with subsequent transfer from the host to Pr³⁺ ions.

[0033] The second PLE spectrum is the excitation spectrum of Mn²⁺emission at a wavelength of 530 nm when the Mn activator atoms arepresent. Again, two excitation bands were observed and those bands areat the same positions as in the system doped solely with Pr. This resultis a clear indication that the emission attributable to the presence ofthe Mn activator is coupled with the quantum cutting transition of thePr³⁺ ions.

[0034] The third PLE spectrum is measured for a system in which only theMn activator ions are present without any Pr. In this system, theabsorption band at a wavelength of 195 nm was completely absent, furtherproving the Mn coupling with the Pr quantum cutting transition. Thesespectra conclusively demonstrate that the observed light emissions inthis phosphor system correspond to those of the energy level design,producing two visible light photons in response to a single UV pumpinglight photon.

We claim:
 1. A method of producing two visible light photons from anoxide-based phosphor doped with praseodymium (Pr) and including atoms ofat least one activator in response to excitation with a singleultraviolet light photon, the method comprising exciting the Pr of theoxide-based phosphor with a photon of ultraviolet light to excite anelectron to an excited state, the excited electron falling to a lowerenergy state in a non-radiative transition and transferring energy toexcite a first activator atom in the oxide-based phosphor, the firstactivator atom emitting a first photon of visible light, and the excitedelectron falling further to a lower energy state in a non-radiativetransition, transferring energy to excite a second activator atom in theoxide-based phosphor, the second activator atom emitting a second photonof visible light.
 2. The method of claim 1 wherein the activator atom isat least one of manganese, terbium, and europium.
 3. The method of claim2 wherein the oxide-based phosphor includes a second dopant chosen fromthe group consisting of magnesium, barium, calcium, zinc, and lanthanum.4. The method of claim 1 wherein the Pr is present in the oxide-basedphosphor in a concentration not exceeding four atomic percent.
 5. Themethod of claim 1 wherein the two visible light photons have the samewavelength.
 6. The method of claim 1 wherein the oxide-based phosphorincludes Al for weakening a crystalline field of the phosphor.
 7. Themethod of claim 6 wherein the oxide-based phosphor is SrAl₁₂O₁₉.
 8. Themethod of claim 7 wherein the activator atom is at least one ofmanganese, terbium, and europium.
 9. The method of claim 7 wherein theoxide-based phosphor includes a second dopant chosen from the groupconsisting of magnesium, barium, calcium, zinc, and lanthanum.
 10. Themethod of claim 7 wherein the Pr is present in the oxide-based phosphorin a concentration not exceeding four atomic percent.
 11. The method ofclaim 7 wherein the two visible light photons have the same wavelength.12. An oxide-based phosphor doped with praseodymium (Pr) and includingatoms of at least one activator and emitting two visible light photonsin response to excitation with a single ultraviolet light photon. 13.The oxide-based phosphor of claim 12 wherein the two visible lightphotons have the same wavelength.
 14. The oxide-based phosphor of claim12 wherein the activator atom is at least one of manganese, terbium, andeuropium.
 15. The oxide-based phosphor of claim 12 wherein theoxide-based phosphor includes a second dopant chosen from the groupconsisting of magnesium, barium, calcium, zinc, and lanthanum.
 16. Theoxide-based phosphor of claim 12 wherein the Pr is present in theoxide-based phosphor in a concentration not exceeding four atomicpercent.
 17. The oxide-based phosphor of claim 12 wherein the phosphorincludes Al to weaken a crystalline field of phosphor.
 18. Theoxide-based phosphor of claim 17 wherein the phosphor includes a latticeof SrAl₁₂O₁₉.
 19. The oxide-based phosphor of claim 18 wherein theactivator atom is at least one of manganese, terbium, and europium. 20.The oxide-based phosphor of claim 18 wherein the oxide-based phosphorincludes a second dopant chosen from the group consisting of magnesium,barium, calcium, zinc, and lanthanum.
 21. The oxide-based phosphor ofclaim 18 wherein the Pr is present in the oxide-based phosphor in aconcentration not exceeding four atomic percent.
 22. The oxide-basedphosphor of claim 18 wherein the two visible light photons have the samewavelength.
 23. A crystalline oxide-based phosphor including Al forweakening a crystalline field of the phosphor, doped with praseodymium(Pr) and containing an activator selected from the group consisting ofmanganese, terbium, and europium and producing two visible light photonsin response to excitation with a single ultraviolet light photon. 24.The crystalline oxide-based phosphor of claim 23 wherein the phosphorhas a lattice of SrAl₁₂O₁₉.